In general, an image recognition system for lane recognition is mounted in an ADAS (Advanced Driver Assistance System) such as an FCWS (Forward Collision Warning System), LDWS (Lane Departure Warning System) or LKAS (Lane Keeping Assistant System).
The lane recognition requires a process of acquiring an image of a target object through a camera, extracting the features of the target object through a digital image processing technique, and checking the target object using the features. At this time, a vanishing point at which two lanes at both sides of a vehicle meet each other serves as very important information for correctly extracting the lane feature.
The vanishing point is varied by a pitch motion of the vehicle or a bend in the road. When the vanishing point is varied, an error may occur in lane recognition information or object acquisition information. Such an error needs to be corrected. Korean Patent Registration No. 10-1257871 discloses a technique that acquires a plurality of images with time differences through a camera, minimizes an error caused by a pitch motion based on a vanishing point and optical flow when the pitch motion occurred, and detects an object. The technique disclosed in this patent document does not directly correct the vanishing point, but reduces an error by minimizing an influence of the pitch motion. However, a variation of the vanishing point needs to be checked, in order to more accurately detect an object.
FIG. 1 illustrates that a vanishing point is varied in a conventional FCWS. In FIG. 1, an image taken by a camera while a vehicle travels on the road 10 shows a forward situation. When a forward vehicle 12 is present in the image, the forward vehicle 12 may be detected through the following process. First, the FCWS acquires a forward image through the camera, and performs inverse filtering on the acquired image in order to distinguish the forward vehicle 12 from other objects. Then, the FCWS recognizes the shape of the forward vehicle 12, and determines whether the forward vehicle 12 is a vehicle. When it is determined that the forward vehicle 12 is a vehicle, the FCWS recognizes the bottom of a rear wheel or the end of a lower shadow area of the forward vehicle 12 as a distance reference point, and calculates a relative distance to the forward vehicle 12 based on the distance reference point.
The relative distance may be obtained through Equation 1 below.
                    Z        =                  λ          ⁢                      h            y                                              [                  Equation          ⁢                                          ⁢          1                ]            
In Equation 1, Z represents the relative distance between an ego vehicle and the forward vehicle, λ represents a focal distance of the camera, h represents the height of the camera, and y represents the height of the distance reference point on the camera.
When the relative distance calculation is repeated in successive images, a relative velocity may be calculated through Equation 2 below.
                    v        =                                            Z                              t                +                                  Δ                  ⁢                                                                          ⁢                  t                                                      -                          Z              t                                            Δ            ⁢                                                  ⁢            t                                              [                  Equation          ⁢                                          ⁢          2                ]            
In Equation 2, v represents the relative velocity of the ego vehicle with respect to the forward vehicle, Zt represents a relative distance at time t, and Zt+Δt represents a variation of the relative distance depending on a time variation Δt. Furthermore, a time to collision (TTC) may be calculated from a value obtained by dividing the relative distance by the relative velocity, and the FCWS warns of a collision risk when the TTC is less than a threshold value as expressed by Equation 3 below.
                    TTC        =                              Z            v                    <                      TTC            threshold                                              [                  Equation          ⁢                                          ⁢          3                ]            
In Equation 3, TTC represents a time to collision, and TTCthreshold represents the threshold value of the TTC.
In Equation 1, y represents a height based on the vanishing point P, and the vanishing point is varied by a pitch motion of the vehicle or a bend in the road. When the vanishing point is varied from P to M as illustrated in FIG. 1, the relative distance must be corrected as expressed by Equation 4 in consideration of the variation of the vanishing point.
                    Z        =                  λ          ⁢                      h                          y              -                              y                m                                                                        [                  Equation          ⁢                                          ⁢          4                ]            
In Equation 4, ym represents a y-axis variation of the vanishing point.
When the variation of the vanishing point is not detected, the FCWS may generate a collision warning even though the forward vehicle is sufficiently separated from the ego vehicle, or may not generate a collision warning even though the forward vehicle is close to the ego vehicle.